Support deserializing flattened untagged enum

[r8d8]

Getting error while flattening enum inside struct:

value: Error("can only flatten structs and maps", line: 6, column: 5)'

Source code: https://gist.github.com/r8d8/2940b5490a90204e5052809eae9b8868

serde version 1.0.51

[dtolnay]

Thanks. Minimized:

#[macro_use]
extern crate serde_derive;

extern crate serde;
extern crate serde_json;

#[derive(Deserialize, Debug)]
struct KdfParams {
    #[serde(flatten)]
    kdf: Kdf,
}

#[derive(Deserialize, Debug)]
#[serde(untagged)]
enum Kdf {
    Pbkdf2 {
        c: u32,
    },
}

fn main() {
    let j = r#" {"c":0} "#;
    println!("{}", serde_json::from_str::<KdfParams>(j).unwrap_err());
}

[dtolnay]

I released Serde 1.0.53 with a fix -- after a cargo update your code should work as written.